1. Field of the Invention
The present invention relates to a voltage/current controller device, particularly for interleaving switching regulators.
Specifically, the invention relates to a controller device as above, which comprises a DC/DC converter having a plurality of modules, each module including a pair of drive transistors connected in series between first and second supply voltage references, a current sensor connected to one transistor in said pair, and a current reading circuit connected to said sensor.
The invention relates, particularly but not exclusively, to a controller device for switching regulators of the interleaving type as used in computer processors, this description making reference to this field of application for convenience of illustration only.
2. Description of the Related Art
As is well known, developments in the electrical characteristics of computer processors, e.g., PC, WORKSTATION, and SERVER, are compelling the manufacturers to seek new solutions in order to meet the requirements of central processing units (CPUs).
In particular, CPUs require an accurately adjusted supply voltage (xc2x10.8% at steady state, xc2x13% in transient conditions).
However, supply voltages as low as 1.1 V, and load currents of up to 100 A, with 100 A/xcexcs edges, are used at present. This requires a higher efficiency than 80%.
So it is that current or voltage control devices must be employed, which can assure of the necessary efficiency. To fill the above demands, a low-cost device of this kind may comprise an interleaving type of DC/DC converter, for example.
In particular, this converter layout is obtained by connecting in parallel N DC/DC converters in a step-down configuration, i.e., with the voltage input and output connected together. Each DC/DC converter is referred to as the xe2x80x9cmodulexe2x80x9d or xe2x80x9cchannelxe2x80x9d.
An interleave configuration needs a synchronization circuit to close the high-side switches of the converter modules with a phase shift equal to the switching period divided by the number N of modules.
For simplicity, reference will be made hereinafter to a DC/DC converter having two interleaving modules.
It should be noted that when a conventional voltage mode control is applied to an interleaving type of converter, an uncontrolled distribution of the currents flowing through the inductors of the parallel modules is produced. Thus, to perform satisfactorily, the converter requires that the total load current be split equally among the modules, i.e., that each module carried a current equal to the target output current divided by N. This control technique is known as xe2x80x9ccurrent sharingxe2x80x9d.
Additionally to said current-sharing option, interleaving DC/DC converters are required to vary the output voltage level proportionally to the target output current. In other words, with Vout,nom being the rated output voltage, i.e., the voltage value when the converter is outputting no current, and lout being the value of the output current, the output voltage level Vout is given as:
Vout=Vout,nomxe2x88x92out*K,
where K is a factor decided upon outside the converter.
This option is known as xe2x80x9cvoltage positioningxe2x80x9d or xe2x80x9cdroop functionxe2x80x9d.
Conventional converter devices with current-sharing and droop function options are available commercially in several different types.
Such options involve the need for a converter operative to read or estimate the output current from each module. In particular, the DC/DC converter is to include a read circuit arranged to read this module current by the voltage drop across an output resistor. This resistor may be parasitic to the circuit, e.g., the power switch resistance Rds,on or the DCR of an inductor, or be an element deliberately introduced in the read circuit and usually designated Rsense.
Using a dedicated resistive element Rsense is advantageous in that the reading obtained is highly accurate and unaffected by temperature (e.g., using resistors made of constantan). It has, however, the disadvantages of being expensive and providing a less efficient current-to-voltage conversion within the converter.
On the other hand, utilizing a parasitic element inside the read circuit is surely more cost-efficient, since existing elements in the read circuit can be used. However, this solution lowers reading accuracy because it responds to both manufacturing variations and operating temperatures.
Illustrated schematically by FIGS. 1 to 4 are different conditions in the operation of an interleaving DC/DC converter according to the prior art.
Assume for simplicity the target output current lout to have been split equally among the N converter modules.
FIG. 1 shows schematically an interleaving DC/DC converter 1 that comprises at least one module 2, in turn comprising a high-side transistor MHS and a low-side transistor MLS connected in series together between a first or supply voltage reference VDD and a second or ground voltage reference GND. The module 2 is connected to a load comprising a network 3, in turn connected between a terminal X intermediate the transistors MHS, MLS and ground GND.
This network 3 comprises a series of an inductor L and a capacitor C.
Illustrated schematically in FIG. 1 is a working condition in which the reading performed is a current reading effected across the drain and source terminals of the high-side transistor MHS.
In this case, the reading is little dissipative. Being Iout,2 the average current from any module 2, i.e., the average current through the inductor L in the network 3, the power dissipated through the DC/DC converter 1 having N modules will be:
D*N*Rds,on*(Iout,2)2
where D is the ratio of the output voltage value Vout to the value of the supply voltage VDD of the DC/DC converter 1 (D=Vout/Vin). The ratio D is, therefore, quite small, in particular between 1 V/12 V and 1.85 V/12 V.
In conventional converters, the high-side transistor MHS will close for a time duration D*Ts (where Ts is the switching period of the converter 1). This duration is very small, however.
Also, when the high-side transistor MHS closes and its source terminal reaches a value equal to an input voltage Vin, the reading becomes injured by noise from capacitive coupling effects.
All this makes for difficult reading.
FIG. 2 likewise shows a working condition in which a current reading is performed across the drain and source terminals of the low-side transistor MLS.
In this case, the reading is little dissipative, and the power dissipated is:
N*Rds,on*(1xe2x88x92D)*[Iout,2]2.
The low-side transistor MLS will close for a time duration (1xe2x88x92D)*Ts. This time allows a reading to be completed even with conventional converters. For example, a resistive element Rsense in series with the low-side transistor MLS may be used.
FIG. 3 shows schematically a working condition in which a current reading is performed across the inductor L of the network 3.
In this case, the reading is dissipative, the power dissipated being:
N*DCR*Iout,22
where DCR is the equivalent resistance of the inductor L in the network 3.
It should be noted, however, that the intermediate node X, being connected to one end of the inductor, would exhibit voltage values within the range of ground reference GND to input voltage Vin. Thus, the reading must be made by filtering the voltage signal at the node X to extract continuous information. This filtering introduces new components, and injures the overall speed of the DC/DC converter 1.
To obviate this, it has been known to use a dedicated resistive element Rsense (not shown) in series with the inductor L.
Likewise in FIG. 4, a working condition in which an input current reading to the DC/DC converter 1 is performed.
In particular, an input terminal IN of the DC/DC converter 1 is connected to an input node XIN through a dedicated resistive element Rsense, with the node XIN being connected to first and second modules, 2a and 2b. These modules 2a, 2b have corresponding high-side transistors MHS, corresponding low-side transistors MLS, and respective networks formed, for simplicity, of a single capacitor C and respective inductors La and Lb.
In this case, the reading is little dissipative. Being Iout,i the average current from the generic i-th module, the power dissipated will be:
Rsense*D*N*(Iout,i)2.
The differential signal across the dedicated resistive element Rsense will always be a low voltage value, but the measuring time will be quite short (equal D*Ts) and compel use of the dedicated resistive element Rsense.
The above discussion of different working conditions clearly shows that a controller with current sharing and droop function options, i.e., adapted for use in an interleaving regulator, can only be obtained when a current reading is performed across the drain and source of the low-side transistor MLS, such as shown in FIG. 2 for example.
An embodiment of this invention provides a voltage/current controller device with appropriate structural and functional features for efficient reading, specifically low-dissipation reading, and that overcomes the limitations of prior devices.
An embodiment of this invention uses a read circuit that can correctly read, with low dissipation, a signal appearing at a suitable sensor and being related to the load on the controller device.
An embodiment of this invention provides a controller device comprising: a DC/DC converter having a plurality of modules, with each module including a drive transistor pair connected in series between first and second supply voltage references, a current sensor connected to one transistor in said pair, and a current reading circuit connected to said sensor. The read circuit comprises a transconductance amplifier connected across the current sensor to sense a voltage signal related to a load current being applied to each module, said transconductance amplifier reading said voltage signal with said transistor in the conducting state.
The features and advantages of a controller device according to the invention will be apparent from the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings.